An ignition plug having a structure in which a center electrode is inserted into an insulator and the insulator is inserted into a metallic shell has been used as an ignition plug for igniting an internal combustion engine such as a gasoline engine. In a process of manufacturing such an ignition plug, an end portion of the metallic shell located on one side along the insertion direction of the insulator is crimped, whereby the metallic shell is fixed to the insulator (see Japanese Patent Application Laid-Open (kokai) No. 2006-66385 “Patent Document 1”). Since such an ignition plug is used in an environment of very high temperature and high pressure, the ignition plug has been demanded to have high seal performance. In order to meet such demand, conventionally, a load applied to the metallic shell for crimping (hereinafter referred to “crimping load”) is increased so as to increase the charging density of talc disposed between the crimp portion and the insulator, to thereby enhance the seal performance of the ignition plug.
However, if the crimping load is increased, the outer shape of a tool engagement portion of the metallic shell located adjacent to the crimp portion thereof deforms, which may cause a problem in that a tool cannot be engaged with the tool engagement portion. The tool engagement portion has a shape (e.g., a regular hexagonal shape) which matches the tool shape as viewed in plan. The tool is engaged with the tool engagement portion when the ignition plug is fixed to an internal combustion engine or the like. Also, if the crimping load is increased, there may arise a problem in that a male screw portion of the metallic shell used to fix the ignition plug to an internal combustion engine or the like increases in length, and fails to establish screw engagement with a female screw portion formed on the internal combustion engine or the like. As described above, when the method of increasing the crimping load is employed, the metallic shell deforms. Therefore, a limit is imposed on enhancement of seal performance.